Each of the methods of fixing it introduces its own unique set of problems and possible instabilities. With PAE, you can have and use more than 4gb of RAM, but individual applications still cannot address more than 4gb of memory.

64bit OSes aren't necessarily faster, and they need different device drivers, which don't exist for legacy devices. There also aren't very many applications that take advantage of 64bit yet.

So... before you go 64bit just for the memory, it would be best to think about what you're really going to gain by doing so, and pick the solution that works best for your real needs.

The thing people seem to overlook is: what is the root cause of their slowness. They just want to say "XYZ is a pile go buy _____".

The machine I'm working right now has Vista Business Edition [for today] and I've got no issues on 2 GB of RAM even when I turn the swapfile completely off. It's as fast as it is with XP Pro. I know this because I spent extensive time reloading xp and vista over and over on this hardware to do disk benchmarking. The biggest system hog was Microsoft Search or Indexer or whatever they call it... It's "superfetch" in Vista. Kill that and your system becomes surprisingly usable. Don't skimp for low end hard disks. E.g. 250 GB Western Digital, 8 MB cache. Disk access is currently a major bottleneck on a lot of desktop systems.

As for which one to buy, only buy the one that YOU are most comfortable to support because ultimately it's going to be you that gets dragged in to troubleshoot it when it has issues.

You do not have to buy software specifically written for Vista if you go Vista. You only have to be concerned about hardware drivers if you go 64-bit. And most times the XP 64-bit stuff will work on Vista 64-bit. I've loaded XP 32-bit drivers in Vista 32-bit on a multitude of hardware and had it work just fine. There's even been cases where a Windows 2000 drivers have been used.

As for which edition to buy, don't spend more for the hyped up editions of Vista or XP. Microsoft has a matrix on their website (and sometimes in stores) that list what each edition gains you. Well surprise, Ultimate says it does something like CD-writing - most people load Roxio, etc. and that works even if the OS doesn't have it built in. Who uses the built in crap anyway??

You do not have to buy software specifically written for Vista if you go Vista. You only have to be concerned about hardware drivers if you go 64-bit. And most times the XP 64-bit stuff will work on Vista 64-bit. I've loaded XP 32-bit drivers in Vista 32-bit on a multitude of hardware and had it work just fine. There's even been cases where a Windows 2000 drivers have been used.

But applications do have to be compiled as 64bit to make use of more than 4gb memory. 64bit OS means the OS has a 64bit address space, 32bit compiled apps still have to run within a 32bit address space. However, that address space isn't just limited to the first 4gb of RAM for those apps, since the OS can map it a virtual address space.

And I agree completely with the slowness things, there's many levels of tweaking you can do. You can also turn off the Aero interface that everyone says looks like OSX. That gives you leaps and bounds of improvement immediately.

Update: Got my new Lenovo X300 laptop in. Runs Vista Business. No issues with speed or stability whatsoever so far. The laptop is frigging awesome for such a small package - 64GB SSD drive, fast as hell.

I'm running Aero too, and there have been no slowdowns or lag of any sort. I'll probably turn it off just to save some battery in the long run (3D operations are more power intensive...)

So, my opinion, try Vista, see if you like it and if it does what you need. It's not extra great, and there's not a compelling reason to upgrade your current computer to it, but your next computer, why not? I'm a skeptic and haven't had any issues yet.